Parents of children with autism said this week's revelation that at least one pharmaceutical company knew of the high levels of mercury in vaccinations years before disclosing it further supports their suspicions that the poison causes neurodevelopmental disorders.

Many parents have long been suspicious of the effects of vaccines containing thimerosal, a compound used to guard against contamination and which is almost 50 percent ethyl mercury. Until recently, the neurotoxin was used in many pediatric vaccines; public health officials first acknowledged the high levels of mercury in those shots in 1999.

On Tuesday, the Los Angeles Times uncovered a 1991 memo from a Merck & Co. vaccinologist -- written to the president of the company's vaccine division -- warning that infants who get their shots on time could receive up to 87 times the recommended daily amount of mercury from fish.

Parents such as Kim Garrison of San Francisco and Jennifer Hoffiz of Danville link that heightened amount of mercury to the problems their children have had, although many doctors say no reputable studies reinforce their claim.

Hoffiz has two children, now 8 and 5. Her daughter, Sabrina, is learning-impaired and her son, Steven, has severe autism. She believes both suffer from mercury poisoning.

"My kids are a perfect example of the least amount of damage mercury can cause to the greatest," said Hoffiz, who runs the Sensory Center in Pleasanton, a program meant to help patients dealing with autism, other neurological disorders and brain injuries through "brain exercises."

Garrison's 12-year-old son, Tod, was diagnosed when he was three. She says no one in their family was autistic, and he was developing normally until he received a combination MMR shot -- for measles, mumps and rubella -- when he was a little over 1 year old.

"I think there is very strongly compelling evidence that my son was poisoned," Garrison said. "If somebody can argue the other way, I would be more than happy to believe them. ... I get angry sometimes that I did what the doctors told me, because I didn't want him to get ill, but I think I could have poisoned my son."

That thinking is common in parents whose children are autistic, according to Bryna Siegel, the director of the Pervasive Developmental Disorders Clinic at UC San Francisco. Siegel, who has acted as an expert witness for pharmaceutical companies being sued by parents, sees no correlation between the vaccinations and autism, pointing toward numerous studies that have found just that.

"You have to balance emotion and science," said Siegel, adding that studies before the vaccines showed autistic children developing normally, then losing skills -- such as language -- at the onset, much like Tod did. "I really worry about people going without vaccines for their kids -- people forget how deadly the diseases are you're preventing."